The oxidation state of the mantle and the extraction of carbon from Earth's interior
The oxidation state of the mantle and the extraction of carbon from Earth's interior
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DOI：
10.1038/nature11679
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发表时间：
2013-01-03
期刊：
影响因子：
64.8
通讯作者：
Frost, Daniel J.
中科院分区：
文献类型：
--
作者：
Stagno, Vincenzo;Ojwang, Dickson O.;Frost, Daniel J.
确定地球硅酸盐地幔的氧逸度是最重要的，因为它影响内部挥发性元素的形态和流动性，并控制着自地球形成以来地球脱气物质的特征（1）。由来自太古代岩石圈的含石榴子石橄榄岩捕虏体记录的氧逸度特别令人感兴趣，因为它们提供了对最古老地幔样品（包括发现金刚石的样品）中活性的含挥发分交代流体和熔体性质的限制（2，3）。在这里，我们报告的实验结果，以测试石榴石氧热压平衡（4，5）在高压条件下有关的最深地幔捕虏体。我们提出了一个最成功的平衡配方，并使用它来确定一个准确的图片，通过超声波岩石圈的氧逸度。最深的岩石的氧逸度被认为是至少一个数量级以上的氧化比以前估计。在金刚石可以形成的深度处，氧逸度与富碳酸盐或富甲烷液体的稳定性不相容，而是与碳酸盐贫乏且由水或硅酸盐熔体主导的交代液体相容。平衡还表明，在绝热减压过程中，含石榴子石岩石的相对氧逸度随深度的减小而增大。这意味着软流圈地幔中的碳将以石墨或金刚石的形式存在，但在上涌过程中，通过还原硅酸盐矿物中的Fe 3+而氧化产生碳酸盐熔体。碳酸盐熔体形成的深度将取决于大块岩石中Fe 3+与总铁的比率。这种“氧化还原熔融”的关系具有重要的意义，开始的可探测的初始熔融和提取二氧化碳从地幔通过减压熔融。
Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet's formation(1). Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found(2,3). Here we report the results of experiments to test garnet oxythermobarometry equilibria(4,5) under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate-or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonatemelt through the reduction of Fe3+ in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe3+ to total iron in the bulk rock. This 'redox melting' relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.